Recently, the sour environment containing hydrogen sulfide (environment of wet hydrogen sulfide) has become to be seen more frequently as the drilling wells for gas and crude oil become deep.
Up to this time, with the flexible fluid transport pipe having, for example, shielding wire around a plastic inner pipe as a metal reinforcing layer, which is used for the transport of, for example, crude oil etc., there has been a problem that water and hydrogen sulfide permeated through the plastic inner pipe stagnate between reinforcing layers and, by the action thereof, hydrogen-induced cracking (HIC), sulfide stress corrosion cracking (SSCC), etc. are caused in the metal reinforcing steel material, though said metal reinforcing layer does not contact directly with inside sour fluid.
With respect to the sulfide stress corrosion cracking there among, it is generally possible to prevent from the occurrence by making the hardness up to HRC 22 (Rockwell C hardness 22) .in the case of carbon steel. For this reason, for the flexible fluid transport pipe to be used in a sour environment, a metal reinforcing material produced in such a way that a low-carbon steel wire material with up to 0.2 wt. % (hereinafter, wt. % is referred to as % simply) of C is worked to a shaped steel wire (flat wire or grooved wire) with a fixed shape in section through the drawing followed by the cold working such as shaped drawing, roller die processing, rolling or the like has been used hitherto as it is or after the low-temperature annealing of up to 500.degree. C. to make the tensile strength up to 80 kgf/mm.sup.2 (tensile strength of 80 kgf/mm.sup.2 is approximately equal to HRC 22).
However, as the partial pressure of hydrogen sulfide rises in the use environment, it has become clear that the hydrogen-induced cracking occurs with the rolled low-carbon steel material.
Conventionally, a production example of steel plate excellent in the HIC characteristic is disclosed in Japanese Patent Publication No. Sho 63-1369. This contains up to 0.12% of C and special elements such as Ti, Mo, Ni, etc. and has the hydrogen-induced crack resistance through the control of rolling conditions. It is however remarkably difficult to assume or apply an art for giving the characteristic of hydrogen-induced cracking resistance to a steel wire with high C accompanying with cold processing from the invention of production method of said steel plate.
Moreover, when using conventional low-carbon steel as said reinforcing material, there has also been a problem that the decrease in strength at welds thereof is remarkable making it impossible to take large design stress.